This invention relates to the manufacture of foundry moulds and cores and to moulding compositions useful therein. More particularly, the invention relates to the use of highly condensed potassium alkali phenolic resins in the binders for the moulds and cores.
Phenol-formaldehyde (PF), phenol-formaldehyde/furfuryl alcohol (PF/FA), urea-formaldehyde/furfuryl alcohol (UF/FA) and furfuryl alcohol-formaldehyde condensation products, catalyzed with strong acid catalysts, such as phosphoric acid, sulphuric acid and paratoluene sulphonic acid and the like, as well known as binders for sand in the production of cold-setting foundry moulds and cores. Aromatic sulphonic acids are more commonly employed than all other types but have the disadvantage that pungent fumes of sulphur dioxide are evolved on thermal decomposition.
UF/FA condensation products contain nitrogen which can form ammonia on thermal decomposition and this tends to neutralize the sulphur dioxide. However, nitrogen in the binder can react with certain metals, such as, for example, grey and nodular irons and steel, resulting in the formation of small bubbles in the final casting, a defect known in the foundry industry as "pinholing". The employment of UF/FA binders is consequently restricted.
Phosphoric acid may be employed as a catalyst but tends to build up on the sand on repeated attrition reclamation and this reduces the refractoriness of the sand. Phosphoric acid is also incompatible with PF/FA condensation products and, as a result, the moulds and cores produced exhibit poor bond strengths.
In the foundry moulding art the use of aqueous highly alkaline phenol-aldehyde resins as binders for sand has not been developed because they tend to produce weak cores. It is known in other fields that the curing of phenol-formaldehyde resins can be catalyzed by esters. The application of this to foundry moulds and cores has been suggested in Japanese Patent publication (Kokai) No. 130627/1975 and copending U.S. application Ser. No. 224,131, filed Jan. 12, 1981.Iadd., now U.S. Pat. No. 4,426,467, issued Jan. 17, 1984.Iaddend.. Whilst these specifications show that foundry cores and moulds having adequate strength and strength increase with time can be made they require the use of relatively high proportions of resin which is costly and makes recovery of the sand, after casting, more difficult.
The present invention is based on the discovery that the use of highly condensed phenol-formaldehyde resins can give moulds and cores which have adequate strength and strength increase at much lower levels of resin. The use of such highly condensed resins in making foundry moulds and cores has not been considered practicable, heretofore.